1. Field of the Invention
The present invention relates to a stud welding apparatus for carrying out automatic welding by using a stud welding gun.
2. Description of Related Art
As this kind of stud welding apparatus, there is conventionally known the following apparatus (see Japanese Published Unexamined Utility Model Registration Application No. 109884/1987). Namely, above a position of disposing a welding jig on which a workpiece can be set, there are disposed a pair of guide frames which longitudinally extend in an X-axis direction at a distance therebetween in a Y-axis direction, where the X-axis and the Y-axis are lines extending in two horizontal directions. There is provided a cartesian coordinate type robot which is made up of a travelling frame which is supported so as to bridge the guide frames and is movable in the X-axis direction, a robot main body which is supported on the travelling frame and is movable in the Y-axis direction, and a vertically elongated elevating arm which is supported on the robot main body and is movable up and down. A stud welding gun is mounted on a wrist portion at a lower end of the elevating arm so that a plurality of welding spots of the workpiece on the welding jig can be automatically welded.
In this conventional stud welding apparatus, there is provided between the travelling frame and the elevating arm an articulated type power supply link which moves to follow the elevating arm so that the electric power can be supplied from a welding power source to the stud welding gun via the power supply link and a power supply passage provided on the elevating arm. In each of the articulated portions of the power supply link there is provided a slip power supply joint which has that contact to be urged by a spring which electrically connects two link members that are to be connected at each of the articulated portions.
Further, in the above-described stud welding apparatus, the wrist portion at the lower end of the elevating arm is made in a double-axis construction provided with a wrist main body which is supported at the lower end of the elevating arm so as to be rotatable about a vertical axis, and a tool holder which is supported on the wrist main body so as to be rotatable about a horizontal axis, and the stud welding gun is mounted on the tool holder. The power supply passage provided on the elevating arm is connected to a power supply passage provided on the wrist main body via a slip power supply joint which is provided on the axis of rotation of the wrist main body and has the same construction as the one described above. The power supply passage provided on the wrist main body is connected to a power supply passage provided on the tool holder via a slip power supply joint which is provided on the axis of rotation of the tool holder and has the same construction as the one described above, and the stud welding gun is thus supplied with the electric power via the power supply passages.
In the above-described stud welding apparatus, the robot main body and the elevating arm are driven by a rack and pinion mechanism. In such a construction, when the welding spots of the workpiece are pressed or forced by the stud welding gun, a pressure reaction force (i.e., a reaction force to be generated against pressing) operates or works on the elevating arm and the robot main body, with the result that a rack and a pinion in the rack and pinion mechanism are likely to be subjected to wear.
Further, in case the above-described slip power supply joint is used, the contact is caused to slidably contact the mating (or the opposite) member when the robot is operated, with the result that the contact and the mating member are subjected to wear. Therefore, the power supply joint must be frequently replaced and the ease with which the apparatus can be maintained becomes poor.